The concept of anarchy is seen as the cardinal organizing category of the discipline of International Relations (IR), which differentiates it from cognate disciplines such as Political Science or Political Philosophy. This article provides an analytical review of the scholarly literature on anarchy in IR, on two levels—conceptual and theoretical. First, it distinguishes three senses of the concept of anarchy: (1) lack of a common superior in an interaction domain; (2) chaos or disorder; and (3) horizontal relation between nominally equal entities, sovereign states. The first and the third senses of “anarchy”’ are central to IR. Second, it considers three broad families of IR theory where anarchy figures as a focal assumption—(1) realism and neorealism, (2) English School theory (international society approach), and (3) Kant’s republican peace. Despite normative and conceptual differences otherwise, all three bodies of theory are ultimately based on Hobbes’s argument for a “state of nature.” The article concludes with a summary of the key challenges to the discourse of international anarchy posed by the methodology of economics and economics-based theories that favor the alternative discourse of global hierarchy.